Methylation effects on the microdomain structures of phosphatidylethanolamine monolayers

The effect of methylated phosphatidylethanolamine derivatives on the ionization properties of signaling phosphatidic acid

Phosphatidylethanolamine (PE) and Phosphatidylcholine (PC) are the most abundant glycerophospholipids in eukaryotic membranes. The differences in the physicochemical properties of their headgroups have contrasting modulatory effects on their interaction with intracellular macromolecules. As such, their overall impact on membrane structure and function differs significantly. Enzymatic methylation of PE's amine headgroup produces two methylated derivatives namely monomethyl PE (MMPE) and dimethyl PE (DMPE) which have physicochemical properties that generally range between that of PE and PC. Additionally, their influence on membrane properties differs from both PE and PC. Although variations in headgroup methylation have been reported to affect signaling pathways, the direct influence that these differences exert on the ionization properties of signaling phospholipids have not been investigated. Here, we briefly review membrane function and structure that are mediated by the differences in headgroup methylation between PE, MMPE, DMPE and PC. In addition, using ³¹P MAS NMR, we investigate the effect of these four phospholipids on the ionization properties of the ubiquitous signaling anionic lipid phosphatidic acid (PA). Our results show that PA's ionization properties are differentially affected by changes in phospholipid headgroup methylation. This could have important implications for PA-protein binding and hence physiological functions in cells where signaling events lead to changes in abundance of methylated PE derivatives in the membrane.

Effect of sphingomyelin headgroup size on molecular properties and interactions with cholesterol

Sphingomyelins (SMs) and sterols are important constituents of the plasma membrane and have also been identified as major lipid components in membrane rafts. Using SM analogs with decreasing headgroup methylation, we systemically analyzed the effect of headgroup size on membrane properties and interactions with cholesterol. An increase in headgroup size resulted in a decrease in the main phase transition. Atom-scale molecular-dynamics simulations were in agreement with the fluorescence anisotropy experiments, showing that molecular areas increased and acyl chain order decreased with increasing headgroup size. Furthermore, the transition temperatures were constantly higher for SM headgroup analogs compared to corresponding phosphatidylcholine headgroup analogs. The sterol affinity for phospholipid bilayers was assessed using a sterol-partitioning assay and an increased headgroup size increased sterol affinity for the bilayer, with a higher sterol affinity for SM analogs as compared to phosphatidylcholine analogs. Moreover, the size of the headgroup affected the formation and composition of cholesterol-containing ordered domains. Palmitoyl-SM (the largest headgroup) seemed to attract more cholesterol into ordered domains than the other SM analogs with smaller headgroups. The ordering and condensing effect of cholesterol on membrane lipids was also largest for palmitoyl-SM as compared to the smaller SM analogs. The results show that the size of the SM headgroup is crucially important for SM-SM and SM-sterol interactions. Our results further emphasize that interfacial electrostatic interactions are important for stabilizing cholesterol interactions with SMs.

On the importance of the phosphocholine methyl groups for sphingomyelin/cholesterol interactions in membranes: a study with ceramide phosphoethanolamine

In this study, we have examined how the headgroup size and properties affect the membrane properties of sphingomyelin and interactions with cholesterol. We prepared N-palmitoyl ceramide phosphoethanolamine (PCPE) and compared its membrane behavior with D-erythro-N-palmitoyl-sphingomyelin (PSM), both in monolayers and bilayers. The pure PCPE monolayer did not show a phase transition at 22 degrees C (in contrast to PSM), but displayed a much higher inverse isothermal compressibility as compared to the PSM monolayer, indicating stronger intermolecular interactions between PCPEs than between PSMs. At 37 degrees C the PCPE monolayer was more expanded (than at 22 degrees C) and displayed a rather poorly defined phase transition. When cholesterol was comixed into the monolayer, a condensing effect of cholesterol on the lateral packing of the lipids in the monolayer could be observed. The phase transition from an ordered to a disordered state in bilayer membranes was determined by diphenylhexatriene steady-state anisotropy. Whereas the PSM bilayer became disordered at 41 degrees C, the PCPE bilayer main transition occurred around 64 degrees C. The diphenylhexatriene steady-state anisotropy values were similar in both PCPE and PSM bilayers before and after the phase transition, suggesting that the order in the hydrophobic core in both bilayer types was rather similar. The emission from Laurdan was blue shifted in PCPE bilayers in the gel phase when compared to the emission spectra from PSM bilayers, and the blue-shifted component in PCPE bilayers was retained also after the phase transition, suggesting that Laurdan molecules sensed a more hydrophobic environment at the PCPE interface compared to the PSM interface both below and above the bilayer melting temperature. Whereas PSM was able to form sterol-enriched domains in dominantly fluid bilayers (as determined from cholestatrienol dequenching experiments), PCPE failed to form such domains, suggesting that the size and/or properties of the headgroup was important for stabilizing sphingolipid/sterol interaction. In conclusion, our study has highlighted how the headgroup in sphingomyelin affect its membrane properties and interactions with cholesterol.

Thermal acclimation of phase behavior in plasma membrane lipids of rainbow trout hepatocytes

The fluorescent probes laurdan (6-dodecanoyl-2-dimethylaminonapthalene) and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl] dipalmitoyl-L-alpha-phosphatidylethanolamine (NBD-PE) in addition to Fourier transform infrared spectroscopy (FTIR) were employed to measure the phase behavior and physical properties of hepatocyte plasma membranes isolated from the livers of thermally acclimated (5 and 20 degreesC) rainbow trout (Oncorhynchus mykiss). The primary objective was to determine the extent to which the phase behavior of membrane lipids is conserved at different growth temperatures. Arrhenius plots of laurdan-generalized polarization revealed a single discontinuity believed to reflect either the onset of the gel-fluid phase transition or the formation of gel phase microdomains, and this discontinuity occurred at significantly higher temperatures in membranes of 20 degrees C (13.2 +/- 0.7 degrees C)- than 5 degrees C (7.2 +/- 0.1 degrees C)-acclimated trout. Similarly, acclimation from 5 to 20 degrees C increased both the onset temperature (from 2.0 +/- 0.3 to 7.2 +/- 0.6 degrees C) and the thermal range (from 10.9 +/- 0.5 to 16.0 +/- 1.0) of the gel-fluid transition as assessed by FTIR. The gel-fluid transition midpoint (approximately -2 degrees C) and completion temperatures (-9 degrees C) were unchanged by thermal acclimation. The anisotropy of NBD-PE fluorescence displayed a distinct minimum in membranes of both warm- and cold-acclimated trout (reflecting alterations in lipid packing that in pure lipid membranes ultimately lead to the formation of nonlamellar phases) in the range of 56-58 degrees C; only membranes of 5 degrees C-acclimated trout displayed an additional minimum at significantly lower temperatures (24.5 +/- 1.7 degrees C). Collectively, these data suggest that the regulation of both the temperature at which gel phase lipids begin to form in response to cooling as well as the propensity of membrane lipids to form nonlamellar phases at higher temperatures may be key features of membrane organization subject to adaptive regulation.

Grafted poly-(ethylene glycol) on lipid surfaces inhibits protein adsorption and cell adhesion

Monolayers of dipalmitoyl-phosphatidylethanolamine (DPPE) mixing with various mole percentages of distearoyl-phosphatidylethanolamine (DSPE)-conjugated poly-(ethylene glycol) (PEG m.w. 750-5000) were deposited on DPPE-coated glass surfaces by the Langmuir-Blodgett method. Increasing percentages of grafted PEG in these supported lipid surfaces increasingly inhibit the adsorption of bovine serum albumin (BSA), laminin, and fibronectin. Increasing percentages of grafted PEG also inhibit the adhesion of erythrocytes, lymphocytes, and macrophages to these supported lipid surfaces. The adsorption of proteins on lipid coated glass surfaces were assayed by the fluorescence of FITC-labelled proteins. Cell adhesion was measured mainly by microscopic counting. The concentration of PEG-grafted lipids required for the inhibition of erythrocyte adhesion decreases with increasing molecular weight of the grafted PEG. The inhibitory effects are strongly dependent on the graft density of PEG at low concentrations, but weakly dependent on graft density at higher concentrations. For DSPE-PEG5000, the change of graft density dependency occurs approximately at the complete coverage of the lipid surface by the grafted polymer in the mushroom conformation (0.7 mol%), and the transition to partial brush conformation. The change-overs become less distinctive for grafted PEG of lower molecular weights, probably due to the failure of strictly mushroom and brush models of the polymer. The relative inhibitory efficiency is protein or cell dependent. The implication on the function of stealth liposomes is discussed.

Cholesterol in condensed and fluid phosphatidylcholine monolayers studied by epifluorescence microscopy

Epifluorescence microscopy was used to investigate the effect of cholesterol on monolayers of dipalmitoylphosphatidylcholine (DPPC) and 1 -palmitoyl-2-oleoyl phosphatidylcholine (POPC) at 21 +/- 2 degrees C using 1 mol% 1-palmitoyl-2-[12-[(7-nitro-2-1, 3-benzoxadizole-4-yl)amino]dodecanoyl]phosphatidylcholine (NBD-PC) as a fluorophore. Up to 30 mol% cholesterol in DPPC monolayers decreased the amounts of probe-excluded liquid-condensed (LC) phase at all surface pressures (pi), but did not effect the monolayers of POPC, which remained in the liquid-expanded (LE) phase at all pi. At low pi (2-5 mN/m), 10 mol% or more cholesterol in DPPC induced a lateral phase separation into dark probe-excluded and light probe-rich regions. In POPC monolayers, phase separation was observed at low pi when > or =40 mol% or more cholesterol was present. The lateral phase separation observed with increased cholesterol concentrations in these lipid monolayers may be a result of the segregation of cholesterol-rich domains in ordered fluid phases that preferentially exclude the fluorescent probe. With increasing pi, monolayers could be transformed from a heterogeneous dark and light appearance into a homogeneous fluorescent phase, in a manner that was dependent on pi and cholesterol content. The packing density of the acyl chains may be a determinant in the interaction of cholesterol with phosphatidylcholine (PC), because the transformations in monolayer surface texture were observed in phospholipid (PL)/sterol mixtures having similar molecular areas. At high pi (41 mN/m), elongated crystal-like structures were observed in monolayers containing 80-100 mol% cholesterol, and these structures grew in size when the monolayers were compressed after collapse. This observation could be associated with the segregation and crystallization of cholesterol after monolayer collapse.

Does cholesterol discriminate between sphingomyelin and phosphatidylcholine in mixed monolayers containing both phospholipids?

The objective of this work was to examine the interaction of cholesterol with both phosphatidylcholines, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and sphingomyelins, N-oleoyl-D-sphingomyelin (O-SPM) or N-palmitoyl-D-sphingomyelin (P-SPM), in monolayers at an air/water interface. We used cholesterol oxidase to probe for the relative strength of sterol-phospholipid interaction, and fluorescence microscopy to visualize lateral domain formation in the mixed monolayers. The ternary mixed monolayers, which contained cholesterol, POPC, and O-SPM had a twofold higher average oxidation rate than the corresponding system containing DPPC and P-SPM. This difference in oxidation rate between saturated and unsaturated systems was observed irrespective of the ratio between phosphatidylcholine and sphingomyelin in the monolayer. With either the saturated or the unsaturated systems, however, the rate of oxidation was influenced by the ratio of phosphatidylcholine to sphingomyelin. As the monolayer content of phosphatidylcholine increased and the sphingomyelin content decreased correspondingly (to maintain a constant cholesterol-to-phospholipid molar ratio), an increase in the average oxidation rate was seen in both saturated and mono-unsaturated monolayer systems. The relationship between the rate of cholesterol oxidation and the phosphatidylcholine/sphingomyelin ratio was not linear, suggesting a preferential interaction of cholesterol with sphingomyelin even when phosphatidylcholine was present in the monolayer. The formation and stability of cholesterol-rich lateral (liquid-condensed) domains in the monolayers, as determined by monolayer fluorescence microscopy, was found to be highly influenced by the phospholipid class, the degree of acyl chain saturation, and by the ratio of phosphatidylcholine to sphingomyelin in the monolayer. The differences in cholesterol oxidation rates and lateral domain formation, as a function of the ratio of two phospholipids in the monolayers, apparently derived from differences in the hydrophobic interactions between the lipids.

Lateral domain formation in cholesterol/phospholipid monolayers as affected by the sterol side chain conformation

The interaction of side-chain variable cholesterol analogues with dipalmitoylphosphatidylcholine (DPPC) or N-palmitoylsphingomyelin (N-PSPM) has been examined in monolayer membranes at the air/water interface. The sterols had either unbranched (n-series) or single methyl-branched (iso-series) side chains, with the length varying between 3 and 10 carbons (C3-C10). The efficacy of interaction between the sterols and the phospholipids was evaluated based on the ability of the sterols to form condensed sterol/phospholipid domains in the phospholipid monolayers. Domain formation was detected with monolayer fluorescence microscopy using NBD-cholesterol as the fluorescent probe. In general, a side chain length of at least 5 carbons was necessary for the unbranched sterols to form visible sterol/phospholipid domains in DPPC or N-PSPM mixed monolayers. With the iso-analogues, a side chain of at least 6 carbons was needed for sterol/phospholipid domains to form. The macroscopic domains were stable up to a certain surface pressure (ranging from 1 to 12 mN/m). At this onset phase transformation pressure, the domain line boundary dissipated, and the monolayer entered into an apparent one phase state (no clearly visible lateral domains). However, with some DPPC monolayers containing short chain sterols (n-C3, n-C4,n-C5, and i-C5), a new condensed phase appeared to form (at 20 mol%) when the monolayer was compressed beyond the phase transformation pressure. These precipitates formed at surface pressures between 6-8.3 mN/m, were clearly observable up to at least 30 mN/m. When the monolayers containing these four sterols were allowed to expand, the condensed precipitates dissolved at the same pressure at which they were formed during monolayer compression. No condensed precipitates were observed with these sterols in corresponding N-PSPM monolayers. Taken together, the results of this study emphasize the importance of the length and conformation of the cholesterol side chain in determining the efficacy of sterol/phospholipid interaction in model membranes. The major difference between DPPC and N-PSPM monolayers at different sterol compositions was mainly the lateral distribution and the size of the domains as well as the onset phase transformation pressure intervals.

Lateral domain heterogeneity in cholesterol/phosphatidylcholine monolayers as a function of cholesterol concentration and phosphatidylcholine acyl chain length

Mixed monolayers of cholesterol and phosphatidylcholines having symmetric, different length acyl chains (10 to 16 carbons each) were prepared at the air/water interface. The partitioning of a fluorescent probe, NBD-cholesterol at 0.5 mol%, among lateral domains was determined by epifluorescence microscopy. The mixed monolayers had cholesterol concentrations of 20, 25, or 33 mol%, and in all these monolayers, lateral domain heterogeneity was observed within a defined surface pressure interval. This surface pressure interval was highly influenced by the phosphatidylcholine acyl chain length, but not by the cholesterol content of the mixed monolayer. The characteristic surface pressure, at which the line boundary between expanded and condensed phases dissolved (phase transformation pressure), and the monolayer entered an apparent phase-miscible state, was about 20 mN/m for di10PC and decreased as a linear function of the phosphatidylcholine acyl chain length to be about 2.5 mN/m for di16PC. During initial compression of the monolayers, the sizes of the condensed phases were generally larger, and to some extent heterogeneous with respect to the size distribution, as compared to the situation in monolayers which had experienced a compression/expansion cycle, which took them above the phase transformation pressure. This suggest that the domains observed during initial compression were not equilibrium structures. This study has demonstrated that both the cholesterol content and the phosphatidylcholine acyl chain length markedly influenced the properties of laterally condensed domains in these mixed monolayers. Since the possibility for the formation of attractive van der Waals forces between cholesterol and acyl chains increase with increasing acyl chain length, and since the phosphocholine head group is similar in all systems examined, the observed differences in domain shapes, properties, and stability most likely resulted from differences in van der Waals forces.

Interaction of free fatty acids with phospholipid bilayers

The partition of free fatty acids (FFA) to egg-phosphatidylcholine (egg-PC) and egg-phosphatidylethanolamine (egg-PE) vesicles was studied. Upon the addition of FFA to the suspension of vesicles, the pH of the aqueous phase changed depending on the length and saturation of the FFA hydrocarbon chain, as well as on the vesicle composition. The medium pH decreased faster if FFA was added to egg-PE as compared to egg-PC vesicles. The fluorescent free fatty acid indicator (ADIFAB) was used to measure the amount of FFA remaining in the aqueous phase. Most of the FFA added to the suspension of egg-PE vesicles remained in the aqueous phase, whereas in the presence of egg-PC vesicles the FFA partitioned preferentially into the lipid phase. The amount of FFA incorporated into the lipid bilayers was estimated by measuring the changes of pH at the lipid bilayer surface, using fluorescein-PE. At high surface concentrations of FFA, decreasing pH at the bilayer surface caused the protonation of FFA, and raised the pK of FFA at the bilayer surface from 5 to about 7. The partition of FFA in egg-PE vesicles was an order of magnitude lower than that in egg-PC vesicles. The incorporation amount was determined more by the molecular packing than by the nature of lipid headgroups, because steroylcaprioyl-PE, which preferred the bilayer structure, behaved more like egg-PC than egg-PE. Understanding FFA partition characteristics would help to interpret the hydrolysis measurements of phospholipids, and to explain many biological activities of FFA.

Monolayer study of mixtures of diacetylenic phosphatidylcholine and phospholipids containing metal-chelating iminodiacetic acid headgroup

The mixing behavior of polymerizable diacetylenic phosphatidylcholine (1) with two diacetylenic phospholipids containing the iminodiacetic acid (IDA) functionality in the headgroup region was studied at the air-water interface. All three phospholipids contained tricosa-10,12-diynoyl acyl chains. In phospholipid 2, the choline group (-CH2-CH2-NMe3) was replaced by an -CH2-CH2-N-(CH2-COOH)2 functionality. In phospholipid 3, the IDA unit was linked to the phosphate headgroup via a sulfonate linker, -CH2-CH2-OS(O2)-O-CH2-CH2-. Monolayers were prepared by mixing polymerizable 1,2 bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (1) with metal-chelating IDA phospholipids (2, 3) on a 10 mM CuCl2 subphase. Studies of monolayer properties of mixtures of 2 and 3 with 1 demonstrated that mixtures of 2 with 1 had better mixing behavior than mixtures of phospholipids 1 and 3.

Molecular organization in phospholipid monolayer domains by correlative fluorescence microscopy and electron diffraction

Lipid monolayer is a half leaflet model for lipid bilayer, which forms the basis of biological membranes. Within a certain range of surface area per molecular of phospholipid monolayers at the air-water interface, where the compressibility was nearly infinite, two phases with different molecular packings were observable by fluorescence microscopy. Mixed-phase monolayers of L-1,2-dipalmitoyl-N-monomethyl-3-phosphatidylethanolamine [DP(Me)PE] or L-1,2-dipalmitoyl-N-dimethyl-3-phosphatidyl-ethanolamine [DP(Me)2PE] were deposited on marker grids coated with Formvar films. The molecular organization in the dark and bright fluorescent areas on the grids was investigated by low dose, selected area electron diffraction. Sharp reflection arcs, at a spacing of 4.2A and arranged in a hexagon pattern, were detected from dark domains of both lipids. A diffuse reflection ring at a spacing of 4.6A was derived from the bright background areas. Diffraction patterns were obtained from neighboring areas along selected dark domains of both lipids. The orientations of diffraction patterns from areas along smooth and curving boundaries of DP(Me)2PE domains were found to turn with the boundaries. In the branching domains of DP(Me)PE, the orientations of diffraction patterns indicated that the branches were formed by twinning. Electron diffraction thus provides an unique way to sample the local molecular packing order and orientation within individual domains in phospholipid monolayers.

Electron diffraction studies of molecular ordering and orientation in phospholipid monolayer domains

The molecular order and orientation of phase separated domains in monolayers of DP(Me)PE and DP(Me)2PE were determined by electron diffraction. Dark and bright fluorescent domains at the air-water interface were observed by fluorescence microscopy. The monolayers were transferred to Formvar coated electron microscope grids for electron diffraction studies. The positions of domains on the marker grids were recorded in fluorescence micrographs, which were used as guide maps to locate these domains in the electron microscope. Selected area electron diffraction patterns were obtained from predetermined areas within and outside the dark domains. Sharp hexagonal diffraction patterns were recorded from dark domains, and diffuse diffraction rings from bright areas in between dark domains. The diffraction results indicated that the dark domains and bright areas were comprised of lipid molecules in solid and fluid states, respectively. The orientation of diffraction patterns from adjacent locations within a dark domains changed gradually, indicating a continuous bending of the molecular packing lattice vector within these domains. Orientation directors in U-shaped DP(Me)2PE domains followed the turn of the arm; no vortex nor branching was indicated by electron diffraction. Directors branching from the "stem" of highly invaginated DP(Me)PE domains usually occurred at twinning angles of n pi/3 from the stem director, which would minimize packing defects in the development of thinner branches. Electron diffraction from local areas of individual domains proved that dark fluorescent domains were solid ones, and that pseudo-long range order existed in these solid domains.